The present invention relates to air-fuel ratio detecting systems, and more particularly the invention relates to improvements in such systems designed to detect the air-fuel ratio of the mixture from the composition of the exhaust gases of an engine.
In the past, an air-fuel ratio detecting system of the type adapted to detect the air-fuel ratio A/F from the composition, e.g., the oxygen content of the exhaust gases from an engine has been proposed which includes an air-fuel ratio sensor principally composed of a metal oxide semiconductor such as titania (TiO.sub.2) semiconductor and having an electric resistance value dependent on the oxygen content and a fixed resistor connected to the air-fuel ratio sensor, whereby the voltage generated at their junction is compared with a fixed reference voltage in a comparator circuit to detect whether the air-fuel ratio of the mixture is greater (oxygen is present in the exhaust gases) or smaller (oxygen is not present in the exhaust gases) than the stoichiometric air-fuel ratio.
A disadvantage of this type of system is that since the reference voltage is preset to a fixed value, if the electric resistance value Re characteristic of the air-fuel ratio sensor shifts on the whole due to the operating temperature or the change in the lapse of time, error will be caused in the detection of air-fuel ratio or the detection will be made impossible.
For instance, a brand-new air-fuel ratio sensor exhibits at an operating temperature of about 500.degree. C. the characteristic shown by a curve x in FIG. 1A and the brand-new sensor shows at an operating temperature of about 800.degree. C. the characteristic shown by a curve y in FIG. 1A, while after an endurance test the air-fuel ratio sensor exhibits at the same operating temperature of 500.degree. C. the characteristic shown by a curve z in FIG. 1B.
As a result, with the brand-new air-fuel ratio sensor used at a temperature of 500.degree. C., a voltage V.sub.A at the junction of the sensor and the fixed resistor changes, as shown by a curve X in FIG. 2. Thus the point of intersection between the voltage V.sub.A and a reference voltage V.sub.S, shown by a solid line, represents a stoichiometric air-fuel ratio ST, making it possible to satisfactorily detect whether the air-fuel ratio of the mixture is greater or smaller than the stoichiometric ratio ST.
However, when the electric resistance value characteristic of the air-fuel ratio sensor changes or shifts due to a change in the operating temperature or by the lapse of time, the voltage V.sub.A changes as for example shown by a curve Y in FIG. 2 and consequently the point of intersection between the voltage V.sub.A and the reference voltage V.sub.S deviates to the greater or lean side of the stoichiometric ratio ST, causing an error in the detection of air-fuel ratio A/F. In an extreme case, there no longer exists any point of intersection for the voltage V.sub.A and the reference voltage V.sub.S, thus making the detection of air-fuel ratio impossible. In particular, as shown in FIG. 2, the voltage corresponding to the air-fuel ratio of mixtures changes very greatly with a change in the ambient temperature when the air-fuel ratio is greater than the stoichiometric ratio, although the voltage corresponding to the air-fuel ratio changes to a relatively small extent when the air-fuel ratio is smaller than the stoichiometric ratio.